Yamaha NP-32 Keyboard Not Turning Off

[foetusmachine]

I watched Dave's recent repair attempt video on the Brymen multimeter where he briefly considered issues with the on/off circuit of the multimeter. I then watched his video from 10 years ago on the soft switch concept which was very helpful.

It got me thinking about an issue I have with a Yamaha keyboard that has a soft switch but won't turn off. The keyboard automatically turns on when power is connected, but when you hold the power button down for ~1 second, it turns off momentarily but turns back on straight away.

Luckily Yamaha has a pretty good service manual and the circuit included. Sure enough, it contains a soft switch that also interfaces with the keyboard, but also with the microprocessor. Below is the circuit that I have added a little detail for ease of reading.

The service manual is not particularly helpful with this particular issue other than to suggest if voltages at test points on the main board are not within spec, the TR200 and TR201 should be investigated/replaced. The test point voltages appear to be okay. The /PSWI and PSWO connections go off to the microprocessor, I presume for functions like auto time-off etc.

Can anyone assist with some guidance on what I should be looking for?

[foetusmachine]

I took a chance with replacing the TR200 PNP Darlington Transistor.

The 2SB1568 wasn't available locally in Sydney in short timeframe (i.e. today), but a similar (higher) spec 2SB1686 was available from Wagner in Sydney and I've just installed it. I got to the shop minutes before their Christmas shutdown.

The issue resolved momentarily. I was able to turn the keyboard on and off as expected, but after a short while the keyboard turned itself on and wouldn't turn off.

[foetusmachine]

Further update.

I hadn't cleaned the flux from around the replacement I undertook and it seems as though it, or something in the flux, was conducting and creating some kind of unintended sub-circuitry. Enough to stuff around with the switching circuit.

Not the first time this has happened to me and I thought I would have learned by now.

I've been able to turn the keyboard on and off and play it in between a number of times now and it's working as expected.

Thanks for everyone's advice! 

[David_AVD]

Check that the switch actually goes low resistance across it when pressed. If not, all sorts of weirdness can happen.

[frisc0]

I had a similar situation with a Yamaha keyboard with soft-key. Issue was that the conducting material on the rubber knob was not conducting anymore. 'Painting' it with conducting ink solved the issue. Switching on only required a very brief connection, for switching off the connection needs to remain closed for 2 secs or so, so if the knob is not conducting constantly, switching off won't happen.